As a premise to control a gain profile in an optical transmission line, it is necessary to know the gain profile as a function of a wavelength. For such purpose, one of two well-known methods is to measure a gain slope using pre-emphasis and the other is to measure a gain slop using an optical filter in a repeater.
The former method is described, for instance, in T. Kawazawa et al., “Novel Gain Tilt Monitoring Method for DWDM Submarine Cable Systems”, OAA 2001, OMD3. In this method, a variable gain equalizer and a photodetector, which measures an optical power after gain equalization, are disposed in an optical repeater. A transmission terminal apparatus increases optical powers of several wavelengths on a short wavelength side (or a long wavelength side) to compare optical power variations of signal lights gain-equalized by the variable gain equalizer in the optical repeater before and after increasing optical power. A gain slope is measured according to the compared results.
The latter method is, for instance, described T. Naito et al., “Active Gain Slope Compensation in Large Capacity, Long-Haul WDM Transmission System”, OAA99, WC5. In this method, an optical repeater comprises a first optical filter to extract wavelength on long wavelength side, a second optical filter to extract wavelength on short wavelength side, a first photodetector to measure an output optical power from the first optical filter, a second photodetector to measure an output optical power from the second optical filter, and a comparator to compare the outputs from the first and second photodetectors. It is determined whether a gain slope is positive or negative by comparing the optical powers of both wavelengths. Even this method cannot measure the gain profile itself.
Although the prior art are capable of measuring whether a gain slope is positive or negative, neither of them can measure a gain profile itself. In the latter method, furthermore, it is required to dispose two optical filters in the optical repeater.
In the latter method, although a gain profile can be measured if a plurality of optical filters or wavelength demultiplexer are disposed, it is required to dispose photodetectors equal to the number of wavelengths to be measured. It is difficult to dispose so many optical elements and circuit elements in an optical repeater.